An important goal in designing a wireless communication system is to increase the number of users that may be simultaneously served by the communication system. This goal may be referred to as increasing system capacity. In an interference limited system, such as a code division multiple access (CDMA) wireless communications system, one way to increase capacity is by lowering the transmit power allocated to each user. By lowering the allocated transmit power, interference for all users is lowered, which provides additional capacity which may be used to add new users.
One way to lower the transmit power for each user is to increase the efficiency of the wireless link or channel between the user or subscriber unit and the base station that serves that user. One phenomena that reduces the efficiency of the communications link is fading. Fading may take several forms, one of which is referred to as multi-path fading. Multipath fading is caused by two or more copies of a transmitted signal combining at the receiver in a way that reduces the overall received signal level.
In the prior art, several diversity techniques have been proposed for reducing the effects of fading. These techniques include orthogonal transmit diversity (OTD) and space-time transmit diversity (STTD).
With reference now to FIG. 1, there is depicted a high-level block diagram of a transmitter and receiver for implementing an orthogonal transmit diversity system. As illustrated, data source 20 provides a stream of symbols, which may be encoded and interleaved. Such symbols may represent data in one or more traffic channels which are to be transmitted to the subscriber unit. Data in the traffic channels may represent voice, data, video, or other data a user desires to transport via the communication system.
The rate that symbols are output from data source 20 is controlled by symbol clock 22. Symbols S.sub.1 and S.sub.2 are shown coming from data source 20 wherein each symbol is output for 1 period of symbol clock 22, or a symbol period which may be described as the duration from T.sub.0 to T.sub.1.
The serial stream of symbols from data source 20 is coupled to commutator 24, which switches at the rate of symbol clock 22. Commutator 24 sends the first symbol to spreader 26, then switches to send the second symbol, S.sub.2, to spreader 28. Subsequent symbols alternate each symbol period between spreader 26 and 28.
Spreaders 26 and 28 spread the symbols by multiplying them by a spreading code, such as a Walsh code. Because the symbol rate at spreaders 26 and 28 is half the rate that symbols are sourced from data source 20, a single Walsh code may be concatenated to form a new Walsh code at spreader 26, and concatenated with an inverted copy to form the spreading code at spreader 28. With these double-length Walsh codes used to spread half-rate symbols, the chip rate output by spreaders 26 and 28 remains the same as a transmission without OTD.
The outputs of spreaders 26 and 28 are coupled to radio frequency transmitters 30 and 32. These radio frequency transmitters may include a modulator, followed by an up converter for up converting the modulated signal to a selected carrier frequency, and an amplifier for providing suitable power for transmitting the radio frequency signal.
The outputs of radio frequency transmitters 30 and 32 are coupled to antennas 34 and 36 for simultaneously transmitting symbols S.sub.1 and S.sub.2. Because antennas 34 and 36 are spaced apart, the characteristics of the various paths or rays that the signals follow from each antenna to the subscriber unit may be measured separately, and described by coefficients shown as r.sub.1, and r.sub.2, where r.sub.1 and r.sub.2 are complex numbers that represent the gain and phase of the channel. Although r.sub.1 and r.sub.2 are treated here as single values, they may be vectors which describe the gain and phase of a plurality of resolvable multipath rays.
Antenna 38 is used by the subscriber unit to receive signals transmitted from antennas 34 and 36. The received signal is down converted an demodulated in down converter and demodulator 40 and decoded in OTD decoder 42.
The output of OTD decoder 42 is recovered symbols multiplied by the square of the magnitude of the channel coefficients r.sub.1 and r.sub.2, respectively. Further details of the operation of OTD decoder 42 are shown in FIG. 2, which is discussed below.
The OTD decoder outputs are coupled to deinterleaver and decoder 44 for the deinterleaving and decoding processes that corresponds to the encoding and interleaving processes performed in data source 20. The output of deinterleaver and decoder 44 is the traffic channel data. Transmit power is reduced for the same quality of service with the OTD diversity technique because different symbols experience different channel gains. This lowers the likelihood that both symbols will simultaneously experience a deep fade. This statistical unlikelihood that both symbols will be faded improves the decoder performance.
With reference now to FIG. 2 there is depicted a schematic representation of OTD decoder 42, which is used in FIG. 1. The input to OTD decoder 42 is a down-converted received signal, which was received from antenna 38. This signal contains traffic channels for all users along with pilot signals that may be used to estimate the channels from each transmit antenna. Channel estimator 50 evaluates the pilot signals and calculates channel coefficients r.sub.1 and r.sub.2.
In a preferred embodiment, despreaders 52 and 54 despread the received signal using a single Walsh code that has been concatenated, as in the transmitter, in order to recover symbols S.sub.1 and S.sub.2. Multipliers 56 and 58 multiply these recovered symbols by the conjugate of the channel estimates in order to compensate for gain and phase changes that occurred in the channel. Decommutator 59 is used to restore the symbol order and thereby double the symbol rate of the outputs from multipliers 56 and 58. The outputs of OTD decoder 42 are the symbols multiplied by the magnitude of the respective channel estimate squared.
With reference now to FIG. 3, there is depicted another method and system for providing transmit diversity. FIG. 3 illustrates a space-time transmit diversity transmitter and receiver. As illustrated, data source 20 and symbol clock 22 provide symbols S.sub.1 and S.sub.2 to space-time coder 60. At the input, S.sub.1 is received by space-time coder 60 during the period from T.sub.0 to T.sub.1. Symbol S.sub.2 is received at the input of space-time coder 60 during the period from T.sub.1 to T.sub.2. Space-time coder 60, which is a special type of transform operation, has two outputs that provide transform signals to two branches of the transmitter.
At the first output of space-time coder 60, symbol S.sub.1 is output during the symbol time from T.sub.0 to T.sub.1, followed by symbol S.sub.2 from symbol time T.sub.1 to T.sub.2. The second output of space-time coder 60 outputs the negative complex conjugate of symbol S.sub.2 during time T.sub.0 to T.sub.1, followed by the complex conjugate of symbol S.sub.1 from the period T.sub.1 to T.sub.2.
The first and second space-time encoded data streams output by space-time coder 60 are then input into spreaders 62 and 64. As shown, spreaders 62 and 64 use Walsh code W.sub.1. Note that the chip rate per symbol remains the same as in the OTD diversity transmitter.
Following the spreading function at spreaders 62 and 64, the spread data streams are modulated, up converted, and amplified by radio frequency transmitters 30 and 32.
The outputs of radio frequency transmitters 30 and 32 are coupled to antennas 34 and 36, which transmit the signals via channels that may be described with channel coefficients r.sub.1 and r.sub.2.
In the subscriber unit, antenna 38 receives the transmitted signals. The transmitted signals are then down converted using down converter and demodulator 40 and coupled to despreader 41, and thereafter to space-time decoder 66. The output of space-time decoder 66 is the estimated symbols multiplied by a factor calculated from the sum of the squares of the magnitude of the channel coefficients. These symbols and factors are then input into deinterleaver and decoder 44, which deinterleaves and decodes the symbols and outputs traffic channel data.
Although deinterleaver and decoder 44 are shown with the same reference numeral in FIG. 1 and FIG. 3 for both the OTD and STTD diversity schemes, respectively, it is important to understand that the deinterleaver and decoder function corresponds to the encoding and interleaving processes used in data source 20. Some performance improvements may be realized by selecting interleaving schemes specifically for a particular one of the diversity techniques. The reason that a different interleaving functions provides a different result is that the OTD diversity scheme uses commutator 24. The interleaving scheme for OTD should be selected so that adjacent symbols experience different fading through different channels.
With reference now to FIG. 4, there is depicted a high-level schematic diagram of a space-time decoder, as used in FIG. 3 at reference numeral 66.
The input to space-time decoder 66 is a down converted and despread received signal that was received from antenna 38. This signal contains traffic channel data for all users along with pilot signals that may be used to estimate the channels from each transmit antenna. Channel estimator 50 evaluates these pilot signals and calculates channel coefficients r.sub.1 and r.sub.2.
Complex conjugators 70 are used as shown to compute the complex conjugate of the down converted and despread signal, and compute the conjugate of channel coefficient r.sub.1 for input to multipliers 72. Multipliers 72 are used to multiply the received signal, or the complex conjugate of the received signal, by channel coefficients r.sub.2, or the complex conjugate of channel coefficients r.sub.1. Adders 74 are used to add the output signals from multipliers 72 to produce signals that represent a symbol multiplied by a factor computed from both channel coefficients. These weighted symbols are then decommutated by decommutator 76 to produce the sequential output of the weighted symbols.
Note that signals labeled x.sub.1 (t) are different they are derived from the signal x(t) at two different times, the times of two sequential symbols periods.
The two methods that are described above for providing transmit diversity use two antennas. Additional transmit diversity may be obtained by increasing the number of antennas. The orthogonal transmit diversity method may be easily implemented with more than two antennas, however merely adding antennas does not increase performance as much as other methods with the same number of antennas.
With regard to the transmitter that uses space-time transmit diversity, this technique is not easily expanded beyond two antennas without using additional system resources, such as Walsh codes or increasing the coding rate, which results in any gain in diversity being cancelled by the loss of capacity.
Therefore, it should be apparent that there is a need for an improved method and system for transmitting and receiving signals transmitted from an antenna array with transmit diversity techniques.